The sale also includes longer routes, with the price of flights loosely tied to distance. Flights begin at $49 each way on Southwest’s shortest routes and increase to $79, $99 or $129 each way for longer flights. Domestic fares included in the sale appear to top out at $154 each way.

The sale launched Tuesday morning and covers travel from Oct. 31 through Dec. 19 and from Jan. 3 through Feb. 14. Flights on Fridays and Sundays are excluded. For Florida and Nevada flights, sale fares are good only Sunday through Wednesday for inbound flights and Tuesday through Friday for outbound flights. (Full fine print)

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The sale fares apply specifically to non-stop options, though many connecting itineraries also show lower-than-usual fares.

Fares on some routes appeared to be even lower than the advertised $49 early Tuesday morning. Some dates between Los Angeles and Albuquerque, for example, were showing on the carrier's website at just $45 each way -- or about $90 round-trip. Some Houston-New Orleans round-trips were available for even less, at about $83.

Southwest’s new international routes are also included in the sale, though those routes come with significantly more restrictions. Fares range from $59 each way (Fort Lauderdale to Belize) to $262 each way (Baltimore-Punta Cana), depending on the route. But those prices are only offered on Tuesdays and Wednesdays. For those who can jump through those day-of-travel hoops, Southwest’s sale covers international flights from Oct. 31 through Dec. 13 and from Jan. 10 through March 2. The sale fares are blacked out for international travel on Nov. 21 and Nov. 22.

Either way, bargain seekers will have to act quickly to snag the fares. The sale ends on Thursday (Oct. 12) at 11:59 p.m. local time in the city of the departing flight. (Full sale details)

The broad fare sale has become a staple for Southwest. It has rolled out similar three-day sales each June and October for the past several years. One sale from June 2015 proved so popular that it crashed Southwest’s website, prompting the carrier to extend that particular sale by an additional 24 hours.

Southwest has used the sales to generate buzz and -- perhaps more importantly -- to sell seats during what are usually some of the slowest travel periods of the year.

In earlier versions of the sales, Southwest had pegged fares to mileage thresholds. Earlier this decade, for example, Southwest priced its sale fares at $49 each way for flights of 500 miles or less and prices increased from there. Flights of 501 to 1,000 miles cost $99 each way and flights of 1,001 to 1,500 miles cost $129 each way. Flights of more than 1,500 miles went for $149 each way.

Southwest ended the precise mileage component of its big sales a few years ago, but its big sales have continued to closely mimic its previous distance-based promotions. The carrier's four advertised sale-fare tiers for this sale — $49, $79, $99 and $129 each way — are similar to the fares Southwest offered on its distance-based sales of years past.

When compared to other fare sales in the U.S., Southwest’s are notable for their broad coverage and liberal availability of its cheapest fares. The latest sale is similar, though there seem to be a few differences compared to the company’s most-recent offerings.

The headlining $49 one-way fares are still broadly offered, but they are noticeably absent from some routes. Southwest’s Boston-Baltimore route, for example, was a reliable $49 (or less) route during many of the airline’s previous sales. But it’s not included in this one. In fact, Southwest’s sale page lists no $49 fares for Boston – though the $59 fare to Nashville is a good value. As is the $72 fare to Dallas Love.

Similarly, Southwest’s signature $49 sale fares do not show up in the carrier’s list of advertised fares for several other markets that commonly see them. Instead, there seems to be a subtle push to slightly higher fare ranges in this latest sale.

Overall, however, the availability of the $49 fares remains remarkably broad. They’ll eventually begin to disappear as they sell out, but another hallmark of the twice-a-year Southwest sale is the seemingly high number of seats that the carrier will sell at its lowest sale fares -- meaning the at least some flights are likely to continue being offered at the lowest price through the end of the sale.

And, of course, there are many longer routes covered in the sale that represent excellent values. A few examples: Atlanta-Denver, $79 each way; Chicago-San Antonio, $99 each way; and Pittsburgh-Los Angeles, $129 each way.

As for the routes where sub-$100 round-trip fares are possible, here's a sampling of some that are included in the sale: